Very large, fracture-bounded polygons occur in Acidalia and Utopia Planitiae, Mars. Two models for the origin of these large polygons postulate drape folding of a sedimentary cover material over an uneven buried surface, with the strain enhancement needed to explain the large size due to differential compaction. Throughout the polygonal terrain are circular grabens inferred to overlie buried crater rims. Drape folding models predict that (1) these circular grabens will bound topographic lows and (2) the surface relief and percent compaction within circular grabens will be proportional to ring diameters. Relief and compaction within 8 circular grabens have been determined by gridding MOLA data in ArcView. All 8 fulfill the first prediction, and 7 of them fulfill the second prediction. These results support a sedimentary origin for polygonal terrain materials, and thus are consistent with the past existence of an 'ocean' in the martian lowland.